refactor: create tasks in coderd instead of frontend (#19280)

Instead of creating tasks with a specialized call to `CreateWorkspace`
on the frontend, we instead lift this to the backend and allow the
frontend to simply call `CreateAITask`.

coderd/database/queries/templateversions.sql

@@ -234,3 +234,10 @@ FROM
 WHERE
 	template_versions.id IN (archived_versions.id)
 RETURNING template_versions.id;
+
+-- name: GetTemplateVersionHasAITask :one
+SELECT EXISTS (
+	SELECT 1
+	FROM template_versions
+	WHERE id = $1 AND has_ai_task = TRUE
+);